The present invention relates to an improved process for catalytic conversion of hydrogen polysulfides to hydrogen sulfide in liquid sulfur and degasification of the liquid sulfur. More specifically, it is concerned with the removal of hydrogen polysulfides and hydrogen sulfide present in liquid sulfur produced by the Claus process. 2. Description of the Prior Art
The toxicity and combustion hazards associated with gaseous hydrogen sulfide are well recognized and documented in literature. Further, the presence of hydrogen sulfide dissolved in sulfur (usually from 200 to 600 ppm by weight, particularly in sulfur produced in a Claus plant or sulfur from certain natural sources), and its associated slow release during subsequent handling and transportation are equally recognized commercially as serious health hazards.
Normally in a gas/liquid system the adsorption rate of the gas is lower at higher temperatures. Thus in principle, the hot liquid sulfure stream in contact with a gaseous phase containing hydrogen sulfide, as found in a Claus plant, should not represent a serious problem if the dissolution is the only adsorption process. However, the hydrogen sulfide is known to combine with the sulfur to form hydrogen polysulfides according to the following reaction: EQU S.sub.x +H.sub.2 S.revreaction.H.sub.2 S.sub.x+1
The formation of the polysulfides is favored at the high temperatures associated with the Claus plant. This is particularly true during the initial oxidation step in the furnace and boiler where the major portion of the sulfur is also produced. Unfortunately, the kinetics of the reverse reaction at lower temperatures characteristic of hydrogen sulfide removal are extremely slow. Thus, the polysulfides are inherently produced in the Claus process, and once formed are extremely slow in decomposing. Consequently, the apparent solubility of hydrogen sulfide in liquid sulfur is unexpectedly high due to the formation of polysulfides, and the subsequent release or removal of hydrogen sulfide is slow and difficult, frequently involving significant quantities of hydrogen sulfide being released days and even weeks after formation.
In response to this problem, commercial specifications have been suggested and adopted setting from 5 to 10 ppm by weight as the maximum desired H.sub.2 S content for safe handling, storage and transportation of bulk quantities of liquid sulfur. To comply with these conditions, it has been recommended that any time the H.sub.2 S content exceeds 15 ppm a H.sub.2 S removal process should be employed.
Various techniques and methods have been proposed to accomplish the removal of H.sub.2 S dissolved in sulfur. In British Pat. No. 1,067,815 a degasification process for removal of sulfur is proposed. The liquid sulfur containing hydrogen sulfide is atomized by forcing it through a jet or nozzle and then the resulting spray is directed against an obstacle, thus promoting the removal of the gaseous H.sub.2 S. It was further disclosed that the presence of ammonia (100 ppm) promoted the removal of H.sub.2 S. In the absence of the use of ammonia the H.sub.2 S reduction is extremely slow, involving long time spans. The use of ammonia inherently results in a contaminated product.
Alternate methods for removal of H.sub.2 S reminiscent of the Claus reaction have been proposed in U.S. Pat. No. 3,447,903 and Canadian Pat. No. 964,040. In U.S. Pat. No. 3,447,903 a catalytic process for producing elemental sulfur from H.sub.2 S and SO.sub.2 in liquid sulfur is disclosed. The catalyst involved is described generically as a basic nitrogen compound having a K.sub.B value (in water) greater than 10.sup.-10 and a solubility in molten sulfur of at least one part per million. This process, as taught, can be practiced for the purpose of controlling purity of liquid sulfur containing small concentrations of H.sub.2 S. Canadian Pat. No. 964,040 involves injecting liquid SO.sub.2 and a nitrogen containing compound, which complexes with the SO.sub.2 to form an adduct, into the molten sulfur for the expressed purpose of having the SO.sub.2 -nitrogen adduct react with the undesirable polysulfide dissolved in sulfur. Hence, it is known that certain nitrogen compounds in combination with SO.sub.2 will catalytically reduce the H.sub.2 S and H.sub.2 S.sub.x concentration found in liquid sulfur. Such processes against inherently involve soluble nitrogen containing species being present in the sulfur after degradation of the sulfide and polysulfides; i.e., the processes merely replace one contaminant for another contaminant.
In a more recent U.S. Pat. No. 3,807,141 an apparatus for reducing the H.sub.2 S and H.sub.2 S.sub.x content of liquid sulfur without the addition of other contaminants such as ammonia or hydrogen sulfide reacting amines is disclosed. The apparatus involves a vertical liquid sulfur scrubbing tower, wherein, the liquid sulfur flows downward through the tower passing from one of a series of L-shaped baffle plates to another which tends to agitate and increase the surface area of the liquid sulfur allowing the dissolved H.sub.2 S to escape. Although the sulfur recovered from the apparatus is free of nitrogen contaminants, the use of this device will involve either 1 to 9 days of continuous recycle or 1 to 8 days of storage prior to passing the sulfur over the series of baffles to insure breakdown of the polysulfides. Such time spans are impractical with respect to contemporary large scale commercial operations.
As summarized in an article entitled "H.sub.2 S Removal from Liquid Sulphur" by F. W. King presented at the November, 1973 meeting of Canadian Natural Gas Processing Association and published in the Energy Processing/Canada, March-April, 1974, the liberation of H.sub.2 S from liquid sulfur takes place in two ways, i.e., through a drop in temperature and through physical agitation. As implied in this article as well as the previously mentioned patents, the extremely slow conversion of hydrogen polysulfide back to hydrogen sulfide prior to degasification is the overall rate limiting step and the primary source of major concern. In addition to the known use of soluble amines and ammonia to catalyze the decomposition of H.sub.2 S.sub.x, an article published by W. J. Rennie entitled "The Removal of H.sub.2 S `Dissolved` in Liquid Sulphur" in the Alberta Sulphur Research LTD. Quarterly Bulletin, v IX, No. 4, January-March, 1973, discloses on a laboratory scale ithe use of alumina, bauxite and Pbs supported on alumina as a solid catalyst for the conversion of H.sub.2 S.sub.x to H.sub.2 S and suggests that they may be useful on a commercial plant scale.